1. Field of the Invention
The present invention relates to a system for determining the position of an object, and in particular to a navigation system for indoor and outdoor use.
2. Description of Prior Art
In the field of outdoor navigation, the technical development of navigation systems using satellite signals has made tremendous progress in the last 25 years. Receivers evaluating the American GPS signal (GPS=Global Positioning System) or the Russian GLONASS signal, e.g. in air traffic or automobile engineering, are in worldwide use. In the past few years, there have also been receivers (GNSS=Global Navigation Satellite System) available in the market that combine both systems and thus obtain higher location resolution. The location resolution attainable can be enhanced further by overlay systems, such as WAAS in Northern America or EGNOS in Europe.
The GPS system as an example of an outdoor navigation system comprises a number of partly moving satellites sending out navigation signals. The position of an object is determined in the receiver of the object on the basis of the transit times of the navigation signals to the receivers and on the basis of the known positions of the satellites. This requires time synchronization of the satellites which is obtained by atomic clocks in the individual satellites. The navigation signals are line-of-sight signals the channel model of which is determined only by the ionosphere of the earth, so that the signal powers at the receiver at all times are essentially the same.
A disadvantage of the GPS system resides in that it requires complex transmitters, in this case satellites, for operating the GPS system as a system for determining the position of an object and as navigation system, respectively. In addition thereto, the individual transmitters must each contain very accurate and complex clocks, in this case atomic clocks, in order to achieve the necessary time synchronization of the transmitters.
An additional disadvantage of the GPS system and of satellite-supported systems in general resides in that they cannot be used for highly accurate navigation in indoor applications, e.g. in buildings, or in outdoor applications in areas shaded off from the satellites, e.g. in “ravines” or rows of high buildings.
In the field of indoor navigation, there are presently suggested solutions for position finding solely in the network and not in the terminal device or receiver, with the position of the receiver being determined by means of the transmitter network of the system and—depending on the application—communicated to the receiver or module to be located by wireless communication. As an example in this respect, mobile communication networks offer navigation services that are also suitable for indoor use. Other systems, based on transponder technologies, are used for logistics purposes and for close-range identification. Locating by distance measurements is the method mostly employed here. Locating takes place by transit time measurement of radio, infrared, ultrasonic or laser signals. More accurate systems also make use of modulated signals in order to obtain higher resolution by means of suitable signal processing. Other methods, such as field strength measurement or the evaluation of the received signal angle in case of more complex base stations equipped with phase-controlled antenna arrangements or phased-array antennas, are mostly used in addition to distance measurement.
The current systems for indoor navigation offer either isolated solutions (pico cells) with tags, solutions based on distance measurement and making use of a bidirectional transmission channel, and solutions with combined systems on the basis of mobile communication services.
A disadvantage of the isolated solutions consists in that the network structure is very complex. A disadvantage of solutions based on distance measurements is that the channel capacity is reached very quickly when there are too many modules or receivers to be located simultaneously in the network or system. A disadvantage of solutions using combined systems consists in that both navigation safety is not ensured at all times as the receiver has to receive at least two base stations or transmitters in the building, and also the locating accuracy still is beyond 25 m in outdoor use with satellite systems and beyond 100 m with a communications infrastructure and, thus, is not suitable for example for indoor applications. Most of the systems for indoor applications offer only 1D locating or distance measurement or 2D locating, e.g. in the form of x-y coordinates.
A particular problem of indoor navigation systems, but also of outdoor navigation systems, e.g. in shaded-off building areas, consists in that indoors, e.g. in buildings, there are high signal dynamics and multipath signals, caused e.g. by reflections of the navigation signals inside the buildings. These multipath signals cause very much distortion to the transmission channel and jeopardize reliable locating or determination of the position of an object.
KRAMAR, E., “Funksysteme für Ortung und Navigation”, Stuttgart, Berlin, Cologne, Mainz 1973, pages 27, 92 to 104, describes the locating systems Loran-A and Loran-C. Locating in case of the Loran-A system is based mainly on the evaluation of an M pulse emitted by a stationary master transmitter and an S pulse emitted by a stationary slave transmitter. The master transmitter first transmits its M pulse. The slave transmitter transmits its S pulse with an offset in time, which is tb+tc, with tb corresponding to the transit time required by the M pulse for covering the distance from the master transmitter to the slave transmitter in order to activate or synchronize the same, and with tc corresponding to a known, fixedly set delay. The difference in transit time between the M and S pulses is measured at a receiver to be located. Knowing the offset in time between the times of transmission and the fixed positions of the master and slave transmitters on the one hand and the offset in time between the reception times of the M and S pulses on the other hand, it is possible to determine a base line on which the receiver has to be present. For differentiating between various master and slave transmitter pairs, the M pulses and the S pulses of various transmitter pairs are transmitted at different repetition frequencies. The Loran-C system differs from the Loran-A system by the number of S pulses used for locating. Each slave transmitter sends out the S pulse with a different offset in time from the M pulse. In order to make the locating signals of master and slave transmitters distinguishable, Loran-C does not transmit individual pulses but pulse groups differing in number. To render various master and slave transmitter groups distinguishable, the pulses of the pulse groups are phase-coded. In addition to coarse locating, as carried out in Loran-A, phase difference measurement is employed for fine locating. For fine locating, the pulses transmitted are provided with a defined envelope having a precisely defined leading edge, on which a specific measuring point or turning point is fixed which can be located precisely in the receiver to be located by differentiation of the leading edge. By provision of several slave transmitters, the position cannot only be determined to an accuracy of a hyperbolic base line, as in case of the Loran-A system, but to the line of intersection of two or more hyperbolic base lines.
U.S. Pat. No. 3,750,178 describes a position detection system for locating the geographical position of a source of discrete radio frequency signals. The system basically operates in the manner of an inverse Loran system. It has a plurality of spaced radio frequency receiving stations at known geographical locations which receive the radio frequency signals of the source to be located, with the differences in transit time of the radio frequency signal from the source to the individual receiving stations being determined. For synchronizing the receiving stations, there is provided a timing transmitter or Loran transmitter which transmits timing signals in order to start digital stop timing devices of the receiver in synchronized manner. The stop timing devices are stopped when the radio frequency signal of the source is received. The digital stop values are communicated from the receiving stations via transmitters to a computer effecting position determination. For precisely determining the time at which the stop timing devices of the individual receiving stations are stopped, the radio frequency signals received are subjected to specific operations in order to obtain marker pulses rendering possible a more accurate determination of the differences in transit time.
DE 21 37 846 B2 describes a modulation phase comparison hyperbolic method and a means for locating surface-bound vehicles. For position locating, there are employed three receiving stations detecting a measuring signal from a vehicle the location of which is to be determined. Location determination is carried out by a central unit to which the receiving stations transmit the received measuring signal via fixed cables. Knowing the known transit times required by the measuring signals for passage over the lines, the central unit can perform the position determination on the basis of the differences in transit time, said unit to this end having counters and a timer, the counters being stopped upon receiving the earliest arriving measuring signal from one of the transmitters. For compensating transit time fluctuations in the participating evaluation and transmission means, the transit times ascertained in the central unit by means of a timer are supplemented by a correction value in a correction memory. The correction value is obtained intermittently from comparison measurements in which a comparison transmitter in a known position, instead of a vehicle, transmits the measuring signal. For the comparison transmitter, the transit times to the central unit via the receiving stations are known so that the difference between the transit time ascertained by the central unit and the known transit times for the comparison transmitter can be stored as correction values in the correction memory. To permit an unequivocal association of a received measuring signal with a specific vehicle or the comparison transmitter, the central unit comprises an encoder and a radio frequency transmitter in order to request by means of a command signal, either a vehicle to be located or the comparison transmitter to issue a measuring signal in the scope of a request prior to the location determination proper. Any vehicle to be located and the comparison transmitter have a vehicle receiver having a selective call receiver that is responsive to a different command signal.
U.S. Pat. No. 4,494,119 describes a distress radio frequency locating method and system combining direction-finding techniques with calculations based on signal strength in order to locate a distress transmitter. The system comprises a distress transmitter to be located, a plurality of slave repeater units arranged at known locations to receive the distress signal and report their signal strength to a central station, a central station to calculate the location of the distress transmitter based on the measured signal strengths and the known locations of the slave repeater units, and to dispatch a rescue unit, and rescue units equipped with direction finding equipment to approach the distress transmitter. The distress transmitter has a target ID associated therewith, and the slave repeater units each have a unit ID associated therewith, all thereof having corresponding encoders. The distress transmitter transmits the target ID along with its distress signal. The slave units report the received signal strength together with the target ID and the unit ID to the central station which, in turn, has a corresponding decoder.
U.S. Pat. No. 5,208,756 relates to a vehicle locating and navigating system operating in conjunction with a cellular telephone network. For navigation of a vehicle, there are provided a vehicle locating transceiver, a standard cellular telephone and an antenna. Via the antenna, the vehicle locating transceiver receives a dual tone multi-frequency or DTMF signal that is transmitted from telephone stations. Based on the known locations of the telephone stations and the received signal strength, the vehicle locating transceiver determines the location of the vehicle. The location ascertained is transmitted via the base stations externally to a central station where e.g. several vehicles are monitored and navigated.
DE 25 25 446 C2 relates to a locating means with highly constant time standard. The locating means consists of a plurality of receiving stations having receivers tunable to the same frequency and highly constant time standards, e.g. an atomic clock, that can be mutually synchronized. A central station ascertains the differences in transit time between two receiving stations each, as well as the location of a transmitter to be located by way of the point of intersection of the hyperbolic base lines corresponding to the differences in transit time ascertained. To perform on the one hand the evaluation of received signals of a transmitter to be located at the central station by way of cross-correlation between two signals received at different receiving stations while, however, providing on the other hand insensitivity to errors during the transmission from the receiving stations up to arrival at the central station, the signals received are provided with a time reference mark derived from a time standard of the receiving stations and transmitted to a central station performing the cross-correlation.
DE 2829558 A1 relates to a hyperbolic phase comparison method for determining the location of surface-bound vehicles and an apparatus for carrying out this method, and thus constitutes in essence a development of the system according to DE 21 37 846 B2 mentioned hereinbefore. To provide for insensitivity to faults in the transmission of the received signals from the receiving stations to the central unit, each measuring signal received at the receiving stations is compared with a locally produced reference signal from a stationary quartz oscillator, and the phase difference from a reference signal is digitally transmitted to the central unit via a radio channel and analyzed there to form the transit time differences. In certain intervals, locating of a calibration transmitter is carried out the phase difference of which at the receiving stations from the locally produced reference signals is known. On the basis of this, correction values are obtained.
DE-OS 1813128 relates to a system for determining and indicating the respective location of vehicles. The system consists of receivers, a central computer and memory and a main clock. Each receiver comprises a clock that are synchronized with the main clock. Synchronization is carried out via delay circuits that effect delays compensating the differences in transit time of the synchronizing signals from the central main clock to the various local receiver clocks. Upon reception of a signal from the vehicle, the receivers transmit the times from the quartz clocks to the central computer carrying out the analysis using the hyperboloid points of intersection.
DE-PS 1240146 also relates to a method of determining the location of vehicles, which does not only use transmit times for location determination, but in which, in addition thereto, also the distance of the particular vehicle from a plane having the stationary stations therein is ascertained and used as correction. It is possible to use receivers as well as transmitters as stationary or fixed stations. The use of accurate time standards is necessary both on the transmitter side and on the receiving side.
DE 19647098 A1 relates to a location system in general, for multiple dimensional locating of an object on the basis of measured transit time differences of electromagnetically transferred signals. According to a first aspect, there is described a location system in which the object to be located may have a receiving or transmitting unit, while the stationary units may be transmitting or receiving units and in which the stationary units may have inaccurate quartz clocks which, for synchronization to a system time, are connected to a common central timing generator. To avoid distortion of the time synchronization due to signal transit times, the use of signal conductors or cables of defined length is suggested, so that the transmission error arising is known and can thus be compensated. In particular, there is described a location system consisting of transmitters for locating a receiver. Each transmitter is connected, via signal conductors of known length, to a central timing generator, so that the transmitters are synchronized by this one timing generator. The transmitters transmit an electromagnetic wave, the carrier wave thereof having a data stream modulated thereupon that contains a time signal as well as a transmitter-defining code that can be correlated. The receiver has a clock generator of its own in order to obtain a local time standard. The receiver utilizes the encoded electromagnetic signals transmitted by the transmitters in order to obtain therefrom, by means of a computing unit, the positional speed and time information in relation to the time standard of their own.